Many photofunctioning materials are known which perform their functions due to irradiation with light or other radiation. These materials can be grouped into classes according to their respective working mechanisms.
For instance, there have been known materials which themselves function as photosensors to accept light energy to be used effectively in a succeeding process, those which themselves undergo a photoreaction to produce useful compounds, e.g., dyes or the like (or to make it impossible to produce useful compounds), and those which, for effective use, undergo a photoreaction to cause remarkable physical changes. Representative materials functioning as photosensors are silver halides in silver salt photography and photoconductors in electrophotography, both being photographic systems having a steadfast position excellent in the image-forming arts. Still, these systems have problems, e.g., such that they require complicated processings for image formation and that they are of complex design when used for full-colored image formation. Therefore, more simplified image-forming methods have been desired.
As for the utilization of photofunctioning materials of the kind which undergo a photoreaction to produce or to destroy useful substances, there are known a color image-forming method in which a radical photographic composition, e.g., one which comprises a diazonium salt or an azide compound, carbon tetrabromide and an aromatic amine, is utilized as a light-sensitive material, an image-forming method utilizing a photoionizing reaction of an organometallic compound or a charge transfer complex, and so on. However, materials belonging to this class have problems in that they are, in general, poor in stability and limited in useful substances to be produced therefrom.
On the other hand, as image-forming systems utilizing a photoredox reaction there have been reported those using the combination of cobalt(III) complexes and photoreducing agents (JP-A-50-139722, JP-A-50-139723 and JP-A-50-139724 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application")), those using the combination of tellurium (IV) compounds and photoreducing agents (JP-A-50-45622 and JP-A-50-150427), those using the combination of copper complexes and photoreducing agents (U.S. Pat. Nos. 3,859,092, 3,860,500 and 3,860,501), and so on. In the photoredox reaction, materials are to remain stable, and the system utilizing the combination of a compound represented by formula (I) in this invention as defined hereinbelow and a photoreducing agent to form a redox couple through the photoredox reaction has more extensive functions than those according to conventional arts.
As for the photofunctioning materials of the kind which cause a remarkable physical change as the result of photoreaction, a wide variety of materials have been known. Examples of photomechanical light-sensitive resins which have been used in practice include systems using a bichromate as a photosensitive material, systems utilizing the photo-crosslinking reaction of polyvinyl cinnamate, systems using a mixture of an azide compound and a novolak resin, systems using the combination of a photopolymerization initiator and a vinyl monomer, systems using a polymeric diazonium salt, systems using the combination of an o-quinonediazide and a novolak resin, systems using a silicone resin into which acryloyl or cinnamoyl groups have been introduced in the side chains thereof, and so on. Besides being used as photomechanical materials, these photosensitive materials can be used as UV hardenable inks, coating materials and so on. Most of the materials belonging to this class are polymerized or crosslinked by the photoreaction to result in conversion into insoluble matters. Contrary thereto, among materials which are converted into soluble matters by optical exposure, or so-called positive-working photosensitive materials, those which are sensitive to UV rays and useful in practice are o-quinonediazides alone at present. Under these circumstances, the emergence of novel positive-working photosensitive materials has been expected.